This invention relates generally to ovens and, more particularly, to convection cooking utilizing halogen lamps.
In thermal and convection ovens, the food is cooked by the air in the cooking cavity, which is heated by a heat source. The heat source in both thermal and convection ovens may, for example, be a sheath heater. Conventional thermal ovens do not use a fan to circulate the hot air in the cooking cavity. Conventional convection ovens, however, include a fan to increase cooking efficiency by circulating the hot air around the food. Specifically, the moving air in convection ovens provide quicker cooking compared to cooking in thermal ovens because in convection ovens, the air movement displaces the boundary layer of air around the food and replaces it with hot air. As a result, the heat transfer from the hot air to the food is more rapid in convection ovens as compared to thermal ovens. In a thermal oven, the boundary layer of air acts as insulation around the food and slows down the heat transfer necessary for cooking the food.
The heat sources utilized in thermal and convection ovens typically require some period of time to heat up to reach the target the temperature, as well as a period of time to cool down when cooking is to cease. Such thermal characteristics of the heat source result in difficulties in precisely controlling oven operation to achieve the desired cooking. For example, if a particular food is to cook at 450 degrees F. for 20 minutes, the oven typically first must be pre-heated to the target temperature. Such pre-heating operation may require at least a few minutes. In addition, once the oven is pre-heated, the food is placed in the oven and cooking proceeds for a period of time, for example 20 minutes. Unless the food is immediately removed from the oven upon expiration of the 20 minute cooking cycle, and even though the heat source may be turned off and is cooling, the heat source continues to generate heat which cooks the food. Therefore, the food may actually cook for more than 20 minutes if it is left in the oven as the heat source cools down.
In an exemplary embodiment of the invention, an oven includes a cooking cavity assembly, a controller, a halogen lamp and fan assembly, and a vent assembly. The cooking cavity assembly includes, for example, a shell and a cooking cavity is located within shell. The controller, in the exemplary embodiment, includes a programmable micro controller coupled to a display, an injection molded knob or dial, and tactile control buttons. Selections are made by rotating the dial clockwise or counter-clockwise and when the desired selection is displayed, pressing the dial. For example, many cooking algorithms can be prestored in the oven memory for many different types of foods. When a user is cooking a particular food item that corresponds to prestored cooking algorithm, the prestored cooking algorithm is selected by rotating the dial until the selected food name is displayed and then pressing the dial. Instructions and selections are displayed on the liquid crystal display.
The oven further includes a halogen lamp and fan assembly, sometimes referred to herein as a convection module. The convection module includes one or more halogen lamps and a fan for circulating heat from the lamps into the cooking cavity.
The vent assembly is provided to facilitate drawing air into and out of the oven. Also, since the exemplary oven is an over the cooktop type oven, the vent assembly is provided for drawing air away from a cooktop located below oven.
The above described oven can be operated in two modes, namely thermal emulation and customized cooking. In the thermal emulation mode, the oven is pre-heated to a target temperature by lamps cycling on and off under the control of the controller. Once the target temperature is reached, the user places the food into the cooking cavity and the food is cooked for the same amount of time as in a conventional thermal oven. For example, if the package directions for a food direct a user to cook at 350 degrees F. for 10 minutes, the oven is preheated until the cooking cavity temperature reaches 350 degrees. The food is then placed into the cavity for 10 minutes.
The halogen lamps are then cycled on and off by the controller to maintain the temperature in the cooking cavity within a tight tolerance around 350 degrees F. Since halogen lamps have a fast response time, once deenergized, the air in the cooking cavity begins to cool. Therefore, although there is some temperature overshoot, e.g., the cavity temperature may reach a temperature higher than 350 degrees F. for some period of time, the cavity begins to cool and significant adverse effects from such overshoot are avoided. If the temperature in the cooking cavity falls below a tolerance temperature, e.g., 340 degrees F. for a 350 degree F. target temperature, then lamps are once again energized. The cycling continues until the cook time expires.
In the customized cooking mode, food is placed in cooking cavity for the entire cooking cycle, including the pre-heat portion of the cycle. A temperature sensor senses the starting temperature of the cooking cavity and controller determines the pre-heat time required to achieve the target temperature. For example, pizza can be pre-programmed into controller. The controller determines, e.g., from a lookup table of preprogrammed cooking times, that pizza cooks at 375 degrees F. for 12 minutes. The user puts the pizza in the oven cavity, selects pizza on the control, and presses start. Based on the sensed starting temperature, the controller calculates the pre-heat time and the amount of cooking that occurs during preheating, and increments or decrements the 12 minute cooking time accordingly.
The convection module including halogen lamps as described above facilitates maintaining the cooking cavity within a narrow temperature band around a target temperature, which facilitates precise cooking. Specifically, precisely controlling when heat is added to the food provides for tight control on cavity temperature because there is less overshoot of the target temperature.
Just as the use of a convection fan increases the speed of cooking through more rapid heating of the food surface, the use of radiant energy also speeds up the cooking of many foods through increased browning of the food surface. The combination of convection and radiant heating provides significant synergy of these two effects, allowing faster cooking than is possible with a thermal oven.